Active high-frequency devices, such as transistors and biased diodes, require a connection to a power supply to operate. The power supply is typically a direct-current (“DC”) power supply, and the bias path from the power supply to the active high-frequency device should provide low impedance at DC, but very high impedance at the frequency of interest. The component used to establish the bias path from the power supply to the active high-frequency device is commonly called a high-frequency “choke.”
An ideal high-frequency choke would consist of a single inductor that provided high impedance over all frequencies of interest. However, the equivalent circuit of a single inductor at high frequencies is a complex LRC circuit due to capacitances between individual turns of the coil and the presence of a surrounding enclosure, which are typically referred to as parasitic capacitances, and series resistance of the wire. This equivalent LRC circuit can have several resonant frequencies within the intended frequency range of use. At certain resonant frequencies, the inductor will appear as a low-impedance path loading the transmission line, resulting in large reflections and transmission loss.
Since simple inductors are not ideal high-frequency chokes, and may have relatively low self-resonate frequencies, they are often limited to narrow-band applications. Consequently, typical chokes may employ several series inductors along with resistors and capacitors to minimize the effect of the aforementioned parasitic capacitances.
Wide-band inductors for use in high-frequency chokes have been developed. One example uses fine, insulated wire wrapped in a conical fashion and the interior is filled with a ferromagnetic material, such as polyiron. In one instance, wire is wrapped around a tapered polyiron core. In another instance, a conical coil is wound around a mandrel, removed from the mandrel, and filled with polyiron-loaded epoxy, which hardens into a solid core. Polyiron is generally iron oxide powder mixed with various polymers to form a non-conductive solid material that is magnetically lossy at high frequencies. Polyiron is used to absorb electromagnetic waves in the frequency range of about 0.5 GHz to 120 GHz.
FIG. 1A shows a side view of a prior art conical inductor coil 10. A lead 12 extends from a narrow end 14 of the conical inductor coil 10 for connection to a microwave circuit (see FIG. 1C, ref. num. 24), and another lead 16 extends from a wide end 18 of the conical inductor coil 10 for connection to bias circuitry (not shown). Insulated magnet wire is typically used to wind the coil, and the ends of the leads 12, 16 are stripped of insulation and soldered to their respective circuits. It is desirable to keep the lead 12 as short as possible. If the lead 12 is too long, the high impedance of the inductor will be transformed (i.e. rotated) to a low-impedance contact at the soldering point and cause large reflections at certain frequencies; however, the lead 12 must be sufficiently long to allow soldering to the microwave circuit.
FIG. 1B shows an end view of the conical inductor coil 10 of FIG. 1A filled with polyiron 20. The polyiron 20 is a tapered core that the conical inductor coil 10 is wrapped around. Alternatively, the conical inductor coil is filled with a liquid resin-polyiron composition that cures to a solid polyiron core inside the conical inductor coil.
FIG. 1C shows a plan view of the conical inductor coil 10 of FIG. 1A electrically soldered to a microwave circuit 24, such as a microstrip circuit. Insulation has been removed from an end 12′ of the lead 12, and the end 12′ is electrically soldered to a center conductor 22 of the microstrip circuit 24 with solder 26.
In order to avoid the problems associated with the length of the lead 12 degrading electrical performance, conical inductor coils have been soldered in a through-hole of an air coaxial transmission line. The stripped end of wire from the narrow end of the conical inductor coil is inserted in the through-hole, and is soldered to the center conductor. Soldering the lead in the through-hole allows the length of the lead to be quite short compared to the end 12′ of the lead 12 shown in FIG. 1C; however, air coaxial transmission lines are difficult to connect to many types of microwave devices, such as thin-film circuits and microwave integrated circuits, that are often included in hybrid microcircuits.
FIG. 2A shows an isometric side view of another prior art inductor coil assembly 30 with a metal end contact 32. A conical coil 34 of magnet wire is wound around a polyiron core 36. The metal end contact 32 is machined from brass or other metal and is pressed directly against the microwave circuit (not shown) with a spring (not shown), thus avoiding the problems arising from soldering the lead to the microwave circuit (see FIG. 1A–1C, ref. num. 12). Bias circuits with such inductor coil assemblies 30 are used in microwave chokes operating up to 50 GHz, and have been shipped in MODEL 8510 network analyzers, available from AGILENT TECHNOLOGIES, INC. of Palo Alto, Calif.
FIG. 2B shows an exploded view of portions of the inductor coil assembly 30 of FIG. 2A. The polyiron core 36 includes a tapered section 38 that the wire of the conical coil is wrapped around. The metal end contact 32 is joined to the polyiron core 36 with an insulator 40 of polyamide. A contact post 42 of the metal end contact 32 fits inside the insulator 40. An end of the wire (not shown) is soldered to the metal end contact 32 and wound around the polyiron core 36, including the portion of the contact post 42 that extends into the polyiron core 36.
Unfortunately, a few turns (typically 3–4) of the wire are wound around the contact post 42, which reduces the inductance of the coil and increases the capacitance of the inductor coil assembly 30 near its tip. Similarly, the metal end contact 32 is relatively large, allowing it to act as a microwave stub at a relatively low frequency, and the large contact area forms a capacitor between the metal end contact 32 and the ground plane of a microstrip circuit. This reduction of inductance and increase in capacitance reduces the self-resonant frequency and operating range of the inductor coil assembly 30.